We seek to understand the molecules and forces that determine the orientation of cleavage planes in dividing frog eggs. This will generate fundamental knowledge relevant to division of all human cells, and may inform on the origin of certain types of birth defect. We will use eggs of the frog, Xenopus laevis, and extracts made from eggs that reconstitute the microtubule and actin based processes that organize early embryos in a cell-free system. Our first aim is to determine the mechanism by which radial arrays of microtubules, called asters, grow to fill the whole egg. Our second aim is to determine how signaling molecules that are recruited to the boundary between two microtubule asters signal to the plasma membrane to position cleavage furrows. Our third aim is to determine the mechanisms by which asters position in the egg, and by which centrioles positions inside asters.